This invention relates to the inspection of fasteners installed in structures, and, in particular, to the inspection of fasteners by analyzing light reflected from a fastener and the portion of the structure surrounding the fastener that reflect light of a predetermined wavelength differently than a coating on the structure.
The fasteners that hold structures together, particularly those structures that are subject to significant dynamic forces and/or pressure over their lifetime, such as aircraft, spacecraft, bridges, vehicles, buildings, ships, train bodies, material handling and storage equipment, ladders, scaffolding, and others, must be properly secured in order to ensure that the structure will perform as intended over its lifetime. For example, fasteners are used to hold together various segments of the structure, such as layers of the structure or overlapping portions of the structure, and/or to attach any type of component to the structure.
For instance, in the aerospace industry, fasteners, such as rivets, are commonly utilized to hold together various segments of an aircraft, such as the wing panels, the fuselage segments, and the wing and tail spar segments. Typically, to comply with aerodynamic requirements, the portion of the fastener that extends beyond the outer surface of the structure is removed such that the fastener is substantially flush with the outer surface of the structure. To remove the portion of the fastener that extends beyond the outer surface of the structure, that portion of the fastener is typically shaved by manual means, automated means, or a combination of both.
An automated fastener installation system, for example, may include both the fastener installation means and the fastener shaving means. As such, the fastener installation system typically has a transfer head on which both the fastener installation means and the fastener shaving means are located. The transfer head can then move with respect to the surface between the installation position, where the installation means is aligned with the area of the structure to receive the fastener, and the shaving position, where the shaving means is aligned with the installed fastener. Once the fastener is shaved, the transfer head moves to align the installation means with the next area of the structure to receive a fastener. One example of such a fastener installation system is the 777 Automated Wing Assembly System, commercially available from the General Electro Mechanical Corporation.
Installing and shaving the fastener, however, produces debris and excess fluid that accumulates on the outer surface of the structure. For instance, when the fastener is installed in the structure, pieces of the structure may be displaced, and may accumulate on the outer surface of the structure. In addition, in order to more easily shave the portion of the fastener that extends beyond the outer surface of the structure, at least one type of lubricant may be sprayed on the fastener. As the lubricant is sprayed on the fastener, and as the fastener is shaved, the lubricant and the shaved pieces of the fastener may also accumulate on the outer surface of the structure.
After an installed fastener is shaved, it is typically inspected to determine whether the fastener is shaved properly. For instance, if the fastener is shaved properly, it is flush with the outer surface of the structure. If, however, the fastener is not shaved enough, i.e., the fastener is high, then a portion of the fastener will extend beyond the outer surface of the structure. In this situation, the fastener may be manually shaved until it is flush with the outer surface of the structure. Alternatively, the automated shaving means may be readjusted and repositioned to align the shaving means with the fastener and to shave more of the fastener. In addition, if the fastener is shaved too much, i.e., the fastener is low, then a portion of the structure surrounding the fastener will also be shaved, which creates a recessed area in the structure that may negatively affect the stability and/or the aerodynamics of the structure. In this situation, extensive rework is typically required to repair the recessed area by filling and blending the area in preparation for a new fastener to be installed. In certain applications, however, such as in aircraft wings, a recessed area that is over 0.005 inches deep from the top of the coating cannot be repaired because the size of the recessed area is too large to ensure structural integrity, even if the recessed area is filled and blended. As such, at least the segment of wing with the recessed area must be replaced, and the fastener(s) reinserted.
The new fastener is then shaved and subsequently inspected. Because a low fastener requires significantly more rework than a high fastener, it is preferable to adjust the shaving means to produce a high fastener, instead of risking shaving off too much of the fastener.
The different parts of the structure in which the fasteners are installed typically have varying rigidities. For instance, the body of an aircraft may be more rigid than the wings of the aircraft. As such, the rigidity of the particular part of the structure in which a fastener is installed may affect the amount of the fastener that is shaved. When the shaving means of the fastener installation system applies the force to the fastener that is required to shave the fastener, the underlying structure may maintain its position if it is sufficiently rigid, or it may move slightly away from the shaving means if it is less rigid. Therefore, if the structure moves slightly away from the shaving means, it will not be shaved enough and will have to be re-shaved at a later time.
The shaved fasteners are typically inspected visually, after removing the fastener installation system from the structure. Fastener inspectors may then run their fingers over the installed fasteners and, if their fingers catch on the fastener or catch in a recessed area, then the fastener is identified as a high or low fastener, respectively. The fastener inspectors may also use a dial gauge during their visual inspection. The dial gauge is a hand held mechanical instrument used to measure linear displacement in the range of thousandths or tens of thousandths of an inch. This inspection process is subject to human error because the inspectors are dependent upon their sense of feel and/or a manual instrument for identification of improperly shaved fasteners. In addition, the visual inspection process is time and labor consuming, and therefore, expensive. Furthermore, some areas of the structure may be difficult and/or dangerous for the inspector to physically reach. For example, for particular aircraft parts, such as the wings and the body of the aircraft, the inspector is required to put on a safety harness and climb to the area of the aircraft to be visually inspected. This can be a dangerous endeavor, not only because of the slanted shape of some of the aircraft parts, but also because the inspector may be standing on the debris and lubricant that has accumulated on the outer surface of the structure during fastener installation and shaving. The debris and lubricant on the outer surface of the structure may be slick, and therefore, the inspector is at an increased risk of slipping during the visual inspection process.
Thus, there is a need in the industry for a technique for inspecting fasteners that are installed in structures without having to interrupt an automated fastener installation system in order to visually inspect each fastener. In addition, there is a need for such a fastener inspection technique that is safer and more efficient than the conventional technique that requires physically inspecting the parts of the structure in which the fasteners are installed by walking to the fasteners over debris and lubricant(s), which is a risky, time consuming and expensive process.